Global Navigation Satellite Systems (GNSS), such as the global positioning system (GPS-system) are used worldwide by users to determine their position (longitude, latitude, altitude) on earth.
The GPS-system comprises a number of satellites orbiting the earth, each satellite transmitting radio signals comprising precise timing information about the time the radio signals are transmitted by the satellite. The radio signals also comprise position information (orbital information) comprising information about the position of the respective satellite, and a satellite identification that is unique for a specific satellite.
Positioning devices, such as GPS-receivers, are arranged to receive these signals and compute their position based on the received signals. Such positioning devices provide absolute positioning information with respect to an absolute frame of reference and may therefore also be referred to as absolute positioning systems.
Positioning devices are arranged to receive these transmitted radio signals and compute the travel time of such a radio signal based on the timing information comprised by the radio signal and a measured time of arrival of the radio signal using a clock comprised by the positioning device. The travel time is usually 65-85 milliseconds. Based on the travel time, the distance of the positioning device to the satellite can be computed, simply by multiplying the travel time with the speed of light (c=299.792.458 m/s).
Based on the received orbital information comprised by the radio signal, the positioning device can compute the position of the satellite. By combining the information of the distance to the satellite and the position of the satellite, the positioning device is placed on an imaginary sphere whose radius equals the distance and whose centre is the satellite.
By repeating this computation process for at least four satellites, the positioning device can compute four of such imaginary spheres, defining one intersection, which defines the position of the positioning device.
Positioning devices are often used in navigation devices comprising digital map data. Such navigation devices may be arranged to show the position as determined on the digital map using a display. Such a navigation device may be referred to as a map displaying device, where the part of the displayed map is determined by the actual position as determined by the positioning device.
Also, such navigation devices may be arranged to compute navigation instructions from a start position (for instance the current position) to a destination position, to guide the user to the destination address. Since the positioning device is able to position the current position on the digital map, the navigation device is capable of providing detailed navigation instructions, such as: “after 100 meters, turn left”. It will be understood that accurate positional information is needed for such applications in order to ensure optimal navigation and optimal user comfort.
In order to increase the accuracy of the position as determined by the positioning device using the absolute positioning system, the positioning device may use more than four satellites. Generally, a positioning device uses information from all satellites from which it receives radio signals. In general, the more satellites are used, the more accurate the determined position.
The accuracy of the position as determined by the positioning device is influenced by a number of factors, such as the computed position of the satellite, the computed travel time of the radio signal, the current time as determined by the clock of the positioning device. A number of techniques are known to decrease the effect of these system errors, such as WAAS (Wide Area Augmentation System) and DGPS (differential GPS), as will be known to a skilled person.
The accuracy of the determined position may be further increased by using a technique called map matching. This technique introduced a further increase in the accuracy of the determined position, by mapping the position as determined to a street or the like as stored in the map database.
However, also a number of further outside errors can be identified reducing the accuracy of the determined position, such as ionospheric effects, errors of the satellite clocks etc. One special type of error is so-called multi-path distortion.
Multi-path occurs in situations in which the radio signal as transmitted by a satellite is reflected by an object, such as a building, and the positioning device receives the radio signal after reflection, possibly together with a not-reflected radio signal. As a result, the computed distance between the satellite and the positioning device introduces an error in the computed position of the positioning device.
Positioning devices may also comprise or interact with a relative positioning system to generally improve the positioning accuracy of the absolute positioning system or to determine a position in situations in which no or not enough radio signals can be received. Relative positioning systems provide local and relative positioning information.
These relative positioning systems may for instance be at least one of a gyroscope, an accelerometer, a compass, a distance meter (such as an odometer), an inclinometer. In case the positioning device is used in a vehicle, such as a car or motor cycle, the relative positioning device may also be a distance/velocity measurement module as usually present in such a vehicle and/or a module detecting steering actions of a steering wheel and/or other sensors that may be present in the vehicle.
Situations in which more emphasis may be put on the relative positioning systems (i.e. information from the relative position system is weighted more heavily), are for instance when the positioning device enters a tunnel or an underground parking. The positioning device will no longer be able to determine its position using the absolute positioning system, as not enough radio signals are received. Inside the tunnel or underground parking, the positioning device uses information received from or obtained with the relative positioning system.
For instance, a gyroscope provides information about relative rotational movement. In combination with the last obtained position based on the absolute positioning system and the distance meter, this may be used to compute a current position within the tunnel or underground parking.
U.S. Pat. No. 5,311,195 describes a navigation system using a combination of an absolute positioning system, such as a GPS-receiver and a relative positioning system, such as an onboard wheel sensor and/or an magnetic compass. According to U.S. Pat. No. 5,311,195 the position as determined by the relative positioning system is updated by the position as determined by the absolute positioning system in case a contour of equal probability of a position determined by the relative positioning system overlaps a contour of equal probability of a position determined by the absolute positioning system. So, in case the accuracy of the relative positioning system is low, the absolute positioning system may be used to update the relative positioning system.
According to the prior art, positioning devices are arranged to determine a position using an absolute positioning system and a relative positioning system, and are further arranged to work                in a first mode, in which the position is determined using the absolute positioning system and possibly the relative positioning system, and        in a second mode, in which the position is determined using the relative positioning system and possibly the absolute positioning system,        
where in the first mode the absolute positioning system is weighted more heavily to determine the position than in the second mode. The positioning devices are arranged to switch from the first mode to the second mode and vice versa, based on determined accuracies of the absolute and/or relative positioning systems.
Based on the above, it is an object to provide a positioning device and method that provides more accurate positional information.